In general, according to a wire rod production process conventionally employed in ironworks, billets as rolling materials (each has a sectional area of 160 mm×160 mm) are first heated in a heating furnace to a rolling temperature of 940˜1200° C. Then, the heated billets are sequentially subjected to a plurality of stages of a rolling process including a rough-rolling stage, intermediate rolling stage(s), a finishing-rolling stage, and the like, so as to produce wire rods having a temperature of 800˜1000° C. and a diameter of 5.5˜42 mm.
Referring to FIG. 1 illustrating a general wire rod production line, once a wire rod W is rolled to have a desired diameter while passing through a finishing-rolling mill 10, the wire rod W is guided to pass through a wire rod guider 20 and a sensor unit 30, which are provided between the finishing-rolling mill 10 and a water cooling device 40 to constitute a detection apparatus 1. Thereby, detection of discontinuous surface flaws of the wire rod W is performed. Thereafter, the wire rod W, having passed through the detection apparatus 1, is primarily cooled to have a temperature of less than approximately 800° C. in the water cooling device 40 and in turn, is secondarily air-cooled to have a temperature of approximately 300˜500° C. by use of atmospheric air while being coiled by means of a conical cooling head 50, so as to produce a coil C.
After being subjected to the rolling process, the wire rod W is moved in one direction by a discharge force of the finishing-rolling mill 10. Also, the wire rod W is coiled to constitute a circular coil C by use of a centrifugal force generated by the conical cooling head 50. In this case, it is unavoidable that the wire rod W have a minimal speed error between the discharge speed of the wire rod W from the finishing-rolling mill 10 and the coiling speed of the wire rod W in the conical cooling head 50, due to wire rod rolling characteristics. This inevitably causes the wire rod W to vibrate in a section between the finishing-rolling mill 10 and the conical cooling head 50.
To solve the above described problem, the wire rod guider 20 having a variety of shapes may be provided at a position for detecting surface flaws of the wire rod W, so as to perform not only a function of guiding the movement of the wire rod passing therethrough, but also a function of alleviating the vibration of the wire rod. Well known examples of the wire rod guider include a pipe type wire rod guider, a roller type wire rod guider, and the like.
The wire rod guider 20 is conventionally configured in such a manner that a wire rod passage thereof has an inner diameter is 10˜20% smaller than an inner diameter of a detection sensor 31 included in the sensor unit 30, through which the wire rod W passes. This configuration has the effects of preventing the vibrating wire rod W from temporarily coming into contact with an inner portion of the detection sensor 31 and preventing damage to the detection sensor 31.
In the case of a pipe type wire rod guider, it shows excessive frictional contact with the vibrating wire rod and thus, suffers from wear of a pipe through which the wire rod is guided and causes surface scratches on the wire rod. For this reason, recently, roller type wire rod guiders, which are more developed than the pipe type wire rod guider, have been arranged at entrance and exit sides of the sensor unit, respectively, to alleviate vibration of the wire rod.
FIG. 2 is a configuration view illustrating a roller-guide type wire rod guider employed in an apparatus for detecting surface flaws of a wire rod according to the prior art. As shown, the prior art wire rod guider 20 includes an entrance roller guide 20a having upper and lower rollers 21 and 22 adapted to externally come into contact with the wire rod W that linearly moves in one direction at an entrance side of the detection sensor 31, and an exit roller guide 20b having upper and lower rollers 23 and 34 adapted to externally come into contact with the wire rod W that linearly moves in one direction at an exit side of the detection sensor 31.
Sensor fixing guiders 25 and 26 are provided at the entrance and exit sides of the detection sensor 31, and more particularly, between the entrance roller guide 20a and the detection sensor 31 and between the exit roller guide 20b and the detection sensor 31, respectively, to accurately guide the movement of the wire rod W.
The entrance and exit roller guides 20a and 20b, through which the wire rod W passes, have an inner diameter smaller than an inner diameter of the detection sensor 31 and an inner diameter of the sensor fixing guiders 25 and 26, to alleviate vibration of the wire rod W caused by a difference in a movement speed of the wire rod W.
Also, the inner diameter of the sensor fixing guiders 25 and 26 is smaller than the inner diameter of the detection sensor 31, to prevent the wire rod W from coming into contact with an inner surface of the detection sensor 31 when the wire rod W vibrates.
However, the wire rod W may inevitably come into contact with not only the upper and lower rollers 21 and 22 of the entrance roller guide 20a provided at an entrance of the sensor unit 30, but also the upper and lower rollers 23 and 24 of the exit roller guide 20b provided at an exit of the sensor unit 30 under specific movement speed and vibration conditions of the wire rod W. Therefore, even if the wire rod W is guided so as not to vibrate while guaranteeing smooth rotation of the rollers 21, 22, 23 and 24, there is a problem in that the wire rod W intermittently shows an extremely deteriorated vibration behavior between the entrance roller guide 20a and the exit roller guide 20b. 
As a result of actively studying the reason of the above described vibration behavior, it has been found that the hot rolled wire rod W has elasticity and ductility and thus, is inevitably subjected to a rotating resistance at a portion thereof that comes into contact with the rollers 21, 22, 23 and 24 in the course of passing through the entrance and exit roller guides 20a and 20b as shown in FIG. 3 and this may cause a movement resistance preventing one-directional movement of the wire rod W.
Accordingly, due to the elasticity and ductility thereof, the wire rod W may vibrate upward and downward following elliptical paths in a sensor section B between the entrance roller guide 20a and the exit roller guide 20b and in an exit guiding section C between the exit roller guide 20b and the water cooling device 40. This causes vibration of the wire rod W. Also, the faster the movement speed of the wire rod W, the greater the vibrating width of the wire rod W.
If a rotating speed of rolling rolls 15 is faster than the movement speed of the wire rod W in the course of moving the rolled wire rod W, having passed through the rolling rolls 15 of the finishing-rolling mill 10, toward the wire rod guider 20, the rolling rolls 15 generate a thrust force that causes the wire rod W to more excessively vibrate upward and downward while following elliptical paths in an entrance guiding section A between the rolling rolls 15 and the entrance roller guide 20a. 
Therefore, if the wire rod W vibrates by the rotating resistance caused by the rollers and the thrust force generated by the rolling rolls, the wire rod W has a maximum vibrating width within the detection sensor 31 that is disposed at the middle of a longitudinal direction of the sensor section B. The excessive vibration of the wire rod W within the detection sensor 31 imparts serious noise to detection results from the detection sensor 31, resulting in deterioration in the reliability of surface flaw detection for wire rod products.
Furthermore, the excessive vibration of the wire rod W within the detection sensor 31 frequently causes damage to the inner portion of the detection sensor 31. In fact, under a specific production condition in that a wire rod having a diameter of 5.5 mm is rolled at a speed of 100˜110 m/s, a normal wire rod detecting operation is impossible and the wire rod suffers from a great amount of surface flaws. As a result, most produced wire rods may have surface flaws and this makes it difficult to commercialize wire rod products.
Meanwhile, referring to FIGS. 4 and 5, the sensor unit 30, which is used to detect surface flaws of the wire rod along with the wire rod guider 20, is shown in detail. As shown in FIGS. 4 and 5, the detection sensor 31 of the sensor unit 30 includes solenoid-type transmitting coils 32, through which an alternating current flows, and solenoid-type receiving coils 33 which are adapted to generate an electric current from a solenoid magnetic field. The detection sensor 31 having the above described configuration acts on the detection of surface flaws of the wire rod W, which moves through the interior of the detection sensor 31 at a high flow rate, on the basis of a variation of an eddy current.
Considering a method for detecting surface flaws of the wire rod W using the detection sensor 31, if an alternating current is applied to the transmitting coils 32, the transmitting coils 32 generate a magnetic field. Thereby, if the wire rod W as a conductor passes through the magnetic field generated by the transmitting coils 32, the magnetic field generated in the coils 32 acts on the wire rod W, thus generating an eddy current over a surface of the wire rod product.
In this case, since the eddy current has an irregular variation due to discontinuous surface flaws generated at the surface of the wire rod product, correspondingly, the eddy current to be applied to the receiving coils 33 of the detection sensor 31 has same irregular variation. The variation value of the eddy current is output on a display unit 39 of a controller that is connected to the detection sensor 31 by use of a cable 35 as shown in FIG. 4. Preferably, for the sake of operator's easy understanding, the variation value of the eddy current is output in the form of a graph.
The detection sensor 31 may experience thermal deformation of a sensor body thereof about a sensor bore 31a when the wire rod W having a high temperature of more than 1000° C. passes through the sensor bore 31a. For this reason, as shown in FIG. 5, the detection sensor 31 contains a cooling water line 34 defined therein. If cooling water is supplied into the cooling water line 34 as a cooling path, the cooling water performs heat exchange with a coil portion 31b in which the transmitting and receiving coils 32 and 33 are arranged by interposing a plurality of partitions 38 therebetween, so as to cool the coil portion 31b. Then, the used cooling water is discharged to the outside.
In the above described eddy current detection method using the detection sensor 31, a load ratio (d/D) of the eddy current acts as a main factor of determining the sensitivity of the eddy current. Here, the load ratio (d/D) represents a ratio of an outer diameter d of the wire rod W to an inner diameter D of a winding of the transmitting and receiving coils 32 and 33, which is, in other words, a distance between the surface of the wire rod W and the transmitting and receiving coils 32 and 33. The shorter the distance between the wire rod W and the transmitting and receiving coils 32 and 33, the more the load ratio of the eddy current increase. This results in an improvement in the sensibility of the detection sensor.
However, the detection sensor 31 of the above described prior art sensor unit 30, as shown in FIG. 5, has a structure in that a cooling water passage 34a is defined between an outer periphery of the sensor hole 31a and an inner surface of the coil portion 31b having the transmitting and receiving coils 32 and 33 to extend parallel to the movement direction of the wire rod W. Consequently, the cooling water passage 34a acts as a factor of reducing the load ratio in relation to an occupancy volume thereof and therefore, there is a limit to improve the sensitivity of the eddy current.
Further, when any impurities contained in the cooling water are attached to or intercept the cooling water line 34, this prevents smooth flow of the cooling water, thus causing deterioration in the cooling efficiency of the cooling water.
Furthermore, when the impurities are attached to the cooling water passage 34a between the sensor hole 31a and the transmitting and receiving coils 32 and 33, the impurities may have an adverse influence on the electric current being applied to the receiving coils 33 during the detection of surface flaws for the wire rod, thus causing deterioration in the accuracy and reliability of detection of the wire rod.
Therefore, the present invention has been made in view of the above problems, and it is a first object of the present invention to provide an air guide type apparatus for detecting surface flaws of a wire rod, which can more stably guide one-directional high-speed movement of the wire rod by alleviating vibration of the wire rod caused by a thrust force of rolling rolls.
It is another object of the present invention to provide an air guide type apparatus for detecting surface flaws of a wire rod, which can reduce not only secondary surface flaws of the wire rod, but also wear of wire rod guiding facilities by allowing the wire rod to come into minimal contact with guiding passages.
It is further another object of the present invention to provide an air guide type apparatus for detecting surface flaws of a wire rod, which can reduce surface flaws of the wire rod and prevent wear and damage to the wire rod and a sensor by alleviating vibration of the wire rod when the wire rod is located at a detecting position within the sensor.
It is another object of the present invention to provide an air guide type apparatus for detecting surface flaws of a wire rod, which can achieve high accuracy and reliability in the surface detection of the wire rod by minimizing noise of a sensor that is used to detect a surface of the wire rod being guided.
It is further another object of the present invention to provide an air guide type apparatus for detecting surface flaws of a wire rod, which can increase a load ratio of an eddy current by reducing a distance between the wire rod to be detected and transmitting/receiving coils to the maximum extent.